1. Field of the Invention
This invention relates to a method and a prosthesis for total ankle replacement.
2. Summary of the Background Information
A surgeon begins a conventional ankle replacement process by making an incision in front of the ankle. Then, portions of the tibia and talus forming the damaged joint surfaces are removed through the incision and the surfaces of the tibia and talus are shaped to accept and hold the mating surfaces of the prosthesis, which is then inserted through the incision for attachment. The distal end of the fibula may be fused to the tibia with a bone graft so that compression forces resulting from the subsequent application of weight to the prosthesis will be carried by the fibula as well as by the tibia. When the surgeon has determined that the prosthesis has been properly installed, the incision is closed.
A conventional ankle prosthesis includes a tibial component and a talocalcaneal component, which have bearing surfaces adjacent to one another that are nearly congruent and cylindrically curved, in a longitudinal direction, parallel to the length of the foot, while the wear surfaces are straight in a lateral direction, across the width of the foot. Lateral movement between the tibial and talus components is limited by flanges extending from one of the components past the wear surface of the other component. Axial rotation may be provided through the use of plates turning against one another. While flexure within the prosthesis in the plantar and dorsal directions is permitted by the curvature of the bearing surfaces, flexure in the inversion and eversion directions is prevented by the shape of the wear surfaces, which are straight in the lateral direction.
Conventional ankle replacement provides the patient with foot movement allowing a nearly normal gait, following recovery. While the patient can walk and perform low-impact exercises, he is cautioned against performing high-impact exercises, such as step aerobics and the use of an inclined treadmill or stairstep machine. The effective life of the prosthesis is dependent on how it is used after recovery, with problems occurring particularly due to loosening of the components from the bone surfaces to which they are attached, requiring surgical correction, possibly with ankle fusion. While such problems are associated with limitations in the bone stock available for the attachment of the prosthetic components, it is known that weakening of bone within the talus results in part from the fact that a conventional talar implant produces high stresses in an annular area of the talus while shielding an area within the annular area from stresses. It is further known that a high percentage of patients with post traumatic or osteoarthritic degradation of the ankle joint also have subtalar advanced disease and related pain, and that subtalar fusion in the presence of a conventional ankle replacement prosthesis increases stress levels, expediting failure. Thus, what is needed are improvements in attachment of the prosthetic components to bone and provisions for increasing the allowed movement between the prosthetic components and for colntolling the motion of the prosthetic components relative to one another to reduce a change for injury.